1. Field of the Invention
The present invention relates generally to a method of crystallizing an amorphous silicon thin film and to a method of fabricating a polycrystalline silicon thin film transistor using the crystallization method. More particularly, it relates to a method of crystallizing an amorphous silicon thin film by forming the amorphous silicon thin film on an insulating substrate and having the amorphous silicon undergo crystallization through heat treatment and application of an electric field, and further to a method of fabricating a polycrystalline silicon thin film transistor using the crystallization method including heat treatment and application of an electric field.
2. Discussion of the Related Art
Many studies have been conducted on polycrystalline silicon thin film transistors because of their high electric field mobility and current driving capability and, especially, the polycrystalline silicon thin film transistors that are commonly used in active matrix liquid crystal display of three-dimensional large integrated circuits. Crystal characteristics of silicon important in polycrystalline silicon thin film transistors since large integrated circuits are formed on the silicon thin film.
The effect of metals in crystallization of an amorphous silicon thin film has been studied. It is, for example, reported that a use of nickel leads to a decrease in the crystallization temperature to 480xc2x0 C. (C. Hayzelden and L. D. Batone. xe2x80x9cSilicide-mediated crystallization of nickel-implanted amorphous silicon thin filmsxe2x80x9d J. Appl. Phys. vol. 73 no. 12 pp. 8279-8289, 1993).
Nickel silicide induces formation and growth of silicon seeds and diffusion of nickel seems to determine the crystallization speed of silicon. In addition, according to the Metal Induced Lateral Crystallization (MILC) method, a polycrystalline silicon thin film, less contaminated with metals can be produced as nickel silicide is diffused with heat and moves towards an area containing no metals.
The Field Assisted Lateral Crystallization (FALC) method has also been studied in relation to crystallization of an amorphous silicon thin film. FALC is a crystallization technique in which a lateral crystallization is accelerated in one direction depending on the polarity of electric field applied. In the FALC of silicon, the lateral crystallization is performed more rapidly in the negative (xe2x88x92) electrode than in the positive (+) electrode (Kyung-Sub Song, Duck-Kyun Choi, xe2x80x9cElectric Field Effect On The Metal Induced Crystallization Of Amorphous Siliconxe2x80x9d, Electrochemical Society Proceedings Volume 97-23, pp75-80).
A section of a sample used in the FALC of silicon is shown in FIG. 1 and its fabrication will be described in connection with the figure as follows.
An oxide layer 10 is deposited as thick as about 5000 xc3x85 on a silicon wafer 100 doped with a first conductivity type impurity. An amorphous silicon thin film 11 to be crystallized is then deposited on the oxide layer 10 by the Plasma-Enhanced Continuous Vapor Deposition (PECVD) method, to a thickness of about 1000 xc3x85. Subsequently, a silicon oxide layer about 1000 xc3x85 thick is deposited on the amorphous silicon thin film and etched by photolithography to form a pattern oxide layer 12 that has a defined profile to selectively expose part of the amorphous silicon thin film 11. As a catalyst in crystallization, a metal thin film 13 consisting of one or a combination of Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Se, Ti, V, Cr, Mn, Cu, Zn, Au, Ag, or alloys thereof is applied to a thickness in the range of 30 to 60 xc3x85 to the whole surface of the exposed substrate, thereby obtaining the sample as shown in the FIG. 1.
After forming an electrode on a part of the sample prepared, an electric field is applied to the electrode during a heat treatment so that the amorphous silicon thin film begins to be crystallized. Thus the metal thin film and a part of the amorphous silicon in contact with the metal thin film are crystallized by metal-induced crystallization during the heat treatment, with the crystallized silicon portion being grown laterally towards another part of the amorphous silicon not contacting the metal thin film. This lateral crystallization proceeds rapidly and uniquely in the negative (xe2x88x92) electrode relative to the positive (+) electrode due to effect of the electric field applied. As a consequence, it is possible to accelerate unidirectional crystallization of silicon by applying the electric field.
If the above-described prior art FALC is applied to the liquid crystal displays that include devices such as transistors fabricated on the insulating substrate, a desirable quality of crystallization could be achieved. Where an amorphous silicon thin film on the substrate of the thin film transistor is crystallized by FALC, a current path is required to be formed in the substrate in order to effect high quality crystallization. By applying an electrical voltage to the amorphous silicon thin film on the silicon wafer, a current path forms over the entire wafer, since the silicon wafer doped with a conductive impurity exhibits electrical conductivity. The current path provides a better crystallization conditions and then improves the crystallization quality of the silicon thin film on the wafer.
Accordingly, the present invention is directed toward a method of crystallizing an amorphous silicon thin film and a method of fabricating a polycrystalline silicon thin film transistor using the crystallization method that obviate the problems of the related art.
An object of the present invention is to provide a method of crystallizing an amorphous silicon thin film, in which a conductive layer is formed between an insulating substrate and the amorphous silicon thin film, and an electric field is applied to cause the crystallization of silicon on the insulating substrate with a current path by FALC, and a method of fabricating a thin film transistor using such a crystallization method.
In an embodiment of the present invention, a method of crystallizing an amorphous silicon thin film includes the steps of: preparing a substrate having a conductive layer; depositing an amphorous silicon thin film on the substrate; forming a metal thin film selectively overlying the amorphous silicon thin film; and performing a heat treatment and application of electric field to the metal thin film.
In another embodiment of the present invention, a method of fabricating a thin film transistor includes the steps of: preparing a substrate having a conductive layer; forming an active layer of amorphous silicon on the substrate; forming a gate insulating layer and a gate electrode on the active layer; doping the active layer with a first conductivity type impurity using the gate electrode as a mask; forming a metal thin film on the whole surface of the substrate including the active layer doped with the impurity; and performing a heat treatment and applying electric field to the substrate including the metal thin film.